1. Field of the Invention
The present invention pertains to the field of fiber-reinforced thermosetting composites. More particularly, the invention pertains to fiber-reinforced prepregs containing thermosetting bismaleimide resin systems containing particulate, functionalized elastomers having a T.sub.g of 10.degree. C. and below, and the composites prepared therefrom.
2. Description of the Related Act
The use of fiber-reinforced thermoset composites continues to grow. While great strides have been made in increasing the strength, toughness, temperature use ceiling, and other important physical properties, improvement is still required. Although some resin systems, e.g. the bismaleimides, offer high use temperatures, epoxy resin systems remain the system of choice for many applications. Cyanate resin systems also are preferred for some applications.
However, all these resin systems are somewhat brittle, and thus easily subject to impact-induced damage. This lack of toughness has limited their use to non-critical applications, e.g. in sporting goods equipment and for non-load bearing applications in the transportation and aerospace industries. Many methods of increasing toughness of such resin systems have been investigated. As one result of such investigations, numerous new epoxy resin monomers have been introduced into the market. However despite initial promise, the use of these new and often higher cost epoxy monomers has not resulted in the increase in the toughness desired in composites. Bismaleimide resin systems are often copolymerized with unsaturated compounds such as diallylbisphenol A for the same reason.
What has become clear from the many investigations is that it is not possible, in general, to predict composite properties based on neat cured resin data, and as a result, air frame manufacturers insist that toughness be assessed on cured composite panels.
Other efforts to improve resin toughness has been the inclusion of soluble thermoplastics or elastomers into the resin system. For example, dissolution of polyethersulfone thermoplastics to epoxy resins was disclosed by Bucknall and Partridge in the British Polymer Journal, v. 15, March 1983 pages 71-75. The systems demonstrating the greatest toughness developed a multiphase morphology upon cure. U.S. Pat. No. 4,656,208 discloses a similar multiphase system wherein a reactive polyethersulfone oligomer and an aromatic diamine curing agent react to form complex multiphase domains. However, the systems of Bucknall have very high viscosities due to requiring excessive amounts of dissolved thermoplastic, and yet still do not meet the desired toughness standards. The systems of U.S. Pat. No. 4,656,208 are capable of preparing composites of good toughness, but are difficult to prepare and to process. In particular, the morphology is very cure-cycle dependent, and variations in the cure cycle may greatly affect the toughness of the cured composite.
The addition of soluble, reactive elastomers is known to increase toughness of epoxy resins, and has been used successfully in epoxy adhesives. However, the addition of soluble elastomers to epoxies for use in fiber-reinforced composites results in a decrease in modulus, strength, and use temperature.
The use of rigid particles to toughen epoxy resins is disclosed in European published application EP-A-O 274 899 where transparent infusible nylon particles when added to epoxy resins which when cured produce an interpenetrating network created an increase in composite toughness, and in European published application EP-A-252725 where fillers of glass and polyvinyl chloride were added to epoxy resin formulations, although the latter appeared to exhibit no increase in toughness as a result of such addition.
In European application EP-A-0 351 027 published Jan. 17, 1990 and in U.S. Pat. Nos. 4,977,218 and 4,977,215, the use of high T.sub.g particulate carboxylated crosslinked elastomers having shore hardness of greater than Shore D50 are said to increase toughness of epoxy resin-based composites. However, it is stated that the particles must remain rigid and perform, in addition to other functions, the function of maintaining ply separation. It is further stated that in order to perform this function, that softer elastomers, or those having a T.sub.g of less than 15.degree. C. will not work. However, it is undesirable to increase interlaminar separation excessively, as this increased thickness of the interlaminar region requires a greater amount of matrix resin. As a result, the volume percent of fiber-reinforcement is decreased and the composite, while being tough, loses strength and modulus. Furthermore, it has been found that such particulate elastomers completely fail to increase toughness of brittle epoxy resin matrices.
U.S. Pat. No. 4,999,238 discloses a multiphase epoxy resin composition containing infusible particles which in turn contain carboxylfunctional elastomers. These compositions are prepared by polymerizing an epoxy resin, a diamine curing agent, e.g. diaminodiphenylsulfone, a reactive polyethersulfone oligomer, and a solution of an elastomer such as B. F. Goodrich Hycar.RTM. 1472. During cure, infusible particles of epoxy/hardener/oligomer/elastomer phase separate, these particles further containing domains of elastomer. However, such multiphasic systems are difficult and expensive to prepare and also difficult to process. Preparation involves multiple process steps which involve use of expensive, functionalized, soluble polyethersulfone oligomers which increase resin system viscosity undesirably. In addition, such systems are subject to unpredictable changes in morphology due to variations in resin processing, cure temperature, and cure cycle.
The same approaches to toughening epoxy resin matrices have also been attempted with other thermosetting resins, with some degree of success. It has been found, however, that often the methods successful in toughening epoxy resins fail with respect to bismaleimide or cyanate resins. An example is the use of particulate thermoplastic polyimide 2080 available from Lenzing AG and Matrimide 5218 thermoplastic polyimide available from Ciba-Geigy. The former was ineffective in toughening epoxy resin matrices but very effective in bismaleimides, while the latter was highly effective in epoxy resins but much less effective in bismaleimides. Therefore, it would be desirable to discover a method of toughening which is to some degree formulation independent.